Radiative efficiency of lead iodide based perovskite solar cells

Abstract

The maximum efficiency of any solar cell can be evaluated in terms of its corresponding ability to emit light. We herein determine the important figure of merit of radiative efficiency for Methylammonium Lead Iodide perovskite solar cells and, to put in context, relate it to an organic photovoltaic (OPV) model device. We evaluate the reciprocity relation between electroluminescence and photovoltaic quantum efficiency and conclude that the emission from the perovskite devices is dominated by a sharp band-to-band transition that has a radiative efficiency much higher than that of an average OPV device. As a consequence, the perovskite have the benefit of retaining an open circuit voltage ~0.14 V closer to its radiative limit than the OPV cell. Additionally, and in contrast to OPVs, we show that the photoluminescence of the perovskite solar cell is substantially quenched under short circuit conditions in accordance with how an ideal photovoltaic cell should operate.

Figure 1. Spectral energy distribution of the two photon fluxes governing photovoltaic energy conversion on earth.

The sun flux, closely originating from a distance diluted 5800 K blackbody source (dashed black line) filtered by the earth atmosphere (solid black line) determines the short circuit current. The earth environment 300 K blackbody flux governs instead the dark saturation current (Magenta dashed line). Note the very large range of absolute values on the Y-axis.

Figure 2. Device layout of the studied solar cells.

(A) Perovskite (MAPI) and (B) Organic (MDMO-PPV:PCBM) cells.

Figure 3. Radiative efficiency of the MAPI cell.

(A) EQE_PV (red) and the spectral emission flux (black) at a current injection condition corresponding to J_sc at 1 sun. The vertical line at the peak of EL marks the direct band gap energy at 1.61 eV. The ratio between the total emitted number of photons and the injected current equals the EQE_EL. (B) The 300 K BB spectra (dashed magenta) multiplied with the EQE_PV provides the photon flux distribution (thick dashed blue line) which, when integrated, corresponds to the dark saturation current in its radiative limit. The identical shape of the spectra, with ~18 magnitudes lower emission flux compared to in (A), certifies the reciprocity relation.

Figure 4. Radiative efficiency of the reference OPV cell.

(A) EQE_PV and spectral emission flux at a current injection condition corresponding to J_sc at 1 sun. The EQE_EL is identified substantially lower for this reference material. The vertical black line, corresponding now to the crossing of the reduced spectra, marks the band gap energy at 1.46 eV. (B) The 300 K BB spectra multiplied with the EQE_PV provides again the photon flux distribution corresponding to the dark saturation current in its radiative limit. The obtained J_0,Rad is just one magnitude higher than for the MAPI cell.

Figure 5. Device PL quenching.

(A) PL of the MDMO-PPV:PCBM at open (black line) and short circuit (red line) conditions highlighting the negligible loading dependence. (B) The MAPI cell displays in contrary substantially different PL quenching behavior under device open (black line) and short circuit (red line) conditions. Inset shows the same data on a linear scale. EL spectra are further included in both graphs to emphasize their spectral position related to PL as well as their spectral shape independence on injection conditions. The units on the Y-axis are no longer absolute, but still correspond to m⁻²s⁻¹eV⁻¹.